Earlier known chair assemblies of this kind are, inter alia, difficult to handle and manipulate, due partly to the unsatisfactory design and positioning of the control means for controlling chair movement and release of the locking means, and partly to the fact that, in order to ensure that the chair will not topple, the carriage structure is of such wide dimensions that it obstructs the personnel in their efforts to move and adjust the position of the chair.
The carriage structure must be given wide dimensions, because if not there is an acute risk that the seat will topple or tip when seating a patient thereon. It is often necessary to seat severely injured patients or handicapped people in the chair. These patients are often taken from a bed, wheelchair or like device and placed initially on the edge of the chair seat, whereupon the chair would immediately tip forwards if the carriage structure were not dimensioned to prevent this from happening.
The risk of the chair tipping backwards is also to be found when adjusting the setting of the chair relative to the carriage structure in order to take certain kinds of X-ray pictures.
A chair assembly for use when taking X-ray pictures and comprising a carriage structure which is overdimensioned in relation to the chair seat and which impedes the X-ray personnel in their work is found described and illustrated in EPC-A1-0 001 407 (Pfizer Inc.).
Other types of chairs intended for quite different purposes and comprising various arrangements for preventing or permitting toppling or tipping of the chair to a given extent are known to the art.
Examples of such chairs are found described and illustrated in SE-B-204 191 (Berggren), FR-B-2 406 435 (Bonneau), U.S. Pat. No. 1,977,550 (Gibb), U.S. Pat. No. 3,075,810 (Kitsopoulos), U.S. Pat. No. 3,368,845 (Watanabe).
U.S. Pat. No. 2,293,324 (Vladeff) describes a radiography process and apparatus comprising a number of hollow travelling frames which are movable relative to each other.